Physiology Lab This Week

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Physiology Lab This Week

• Print out Powerpoints on Vision Part 2 and
  Vestibulo-cochlear.
• Sensory Physiology Part 3Two point
  discrimination on back of hand, fingertip, cheek,
  and calf.

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Sensing the world

• Sensory coding sensory circuits code for
  modality, intensity, location, and duration of
  external stimuli.
• Transduction the conversion of a physical
  stimulus into a change in membrane potential
  (electrochemical signal)
• Signals are transmitted in the form of graded
  potentials, action potentials, and synaptic
  interaction
• Receptors cells that will respond to specific
  stimuli and perform transduction
• The process of sensory coding starts here
• Specificity receptors are often sensitive to
  specific stimuli varies with receptor type
  (adequate stimulus)

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Chemosensory signals

• 300 types of olfactory protein receptors in the
  human system
• 5 major gustatory or taste receptors

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S 3

• Receptors for gustation

5 different types of receptor proteins (but not
all in the same cell)
Receptor cell
1st order sensory neuron
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Somatosensory and the sensation of touch

• Skin is largest sense organ up to 2 million
  receptors
• What is occurring during transduction?
• What is the full repertoire of sensations from
  skin?

Ok, something sweet
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Somatosensory sensation of touch, vibration,
pain, and temperature
Sustained stimulus
Fluctuating stimulus
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S 9
This diagram is misleading Different types of
receptors are NOT part of the same sensory neuron!
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S 10
Labeled Lines Different sensory modalities are
transmitted separately along distinct pathways.
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Most mechanoreceptors (touch fibers) are similar
it is the environment around the neuron the
varies
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Receptors and transduction
Activation of mechanically gated channels
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Receptor potentials
e.g. somatosensory
e.g. gustatory

• Def the graded potentials that are the direct
  result of transduction within a receptor cell or
  receptive membrane. Transduction leads to a
  receptor potential.
• Amplitude of the receptor potential is usually in
  proportion to the stimulus intensity.
• Receptive membranes at distal tips of sensory
  axons (somatosensory and olfactory systems have a
  trigger zone and thus action potentials.
• Other receptors are short, specialty cells with
  no axon (visual, gustatory, auditory, and
  vestibular systems). The graded receptor
  potentials will directly change amount of NT
  secretion.

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Sensations to touch Mechanoreceptors contain
receptor proteins that respond to stretching,
distortion, or pressure on the peripheral plasma
membrane
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• NT is released in quanta
• The amount of quanta released depends upon the
  frequency of APs across the axon terminal membrane

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Fig. 07.05
Activation of mechanically gated channels and
convergence of graded receptor potentials from
different parts of the dendritic arbor of the
receptor. Thus a receptor cell with a more
extensive arbor will likely be more ____________.
Why?
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Receptive Fields

• An individual receptor will be activated by
  stimuli that fall within a specific area
  receptive field (RF)
• RF size varies depending upon the dendritic arbor
  of the individual receptor and position of the
  receptor in the sensory organ
• RF is usually larger than arbor
• Where would you find small receptive fields and
  where would you find large fields?

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Receptive fields of secondary afferents depend
upon amount of convergence in the circuit
So where do you expect to see large amounts of
convergence and where do expect to see low
amounts?
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RF?
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Fig. 07.06
Two-point discrimination test

• The size of receptive fields, amount of
  convergence (both within receptor and the
  circuit), and level of overlap will determine the
  resolution of the sensory modality and our
  ability to spatial discriminate sensations.

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Convergence and two-point discrimination test
What about amount of overlap? How might that
affect our ability to spatially resolve a
stimulus?
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Fig. 07.08
Receptive fields often overlap how might the
neural circuit enhance differences and thus
spatial discrimination?
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Spatial discrimination is enhanced by lateral
inhibition

• Lateral inhibition involves near neighbors that
  inhibit each other
• Lateral projections inhibit via NTs and IPSPs

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Fig. 07.11
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Relative timing of stimuli

• Receptors show different state of temporal
  adaptation (Shouldnt that be acclimation, Dr
  Davis?)
• Tonic vs. phasic
• Tonic receptors show little adaptation and
  continue to transmit signals as long as there is
  a stimulus
• Phasic receptors show a high level of adaptation
  and will decrease their responsiveness to a
  steady stimulus